Patent GB-468208-A (GB)
Title: Electrical testing apparatus
IPC: G01N27/416
Filed: 1935-10-30
Abstract: 468,208. Current and pressure meters; potentiometers. NATIONAL TECHNICAL LABORATORIES. Oct. 30, 1935, No. 29983. Convention date, Sept. 21. [Class 37] A testing - apparatus providing a range of positive and negative. potentials comprises a resistance 89b, Fig. 2, with a relatively movable sliding contact 87b, means such as a battery 91b for impressing a predetermined potential across the ends of the resistance, a pair of terminals for connection with an E.M.F. to be measured, one of the terminals being connected to the sliding contact 87b, and means for impressing a fixed reference potential on the other terminal, the said means including resistance elements 92b, 93b, shunting the resistance 89b, the other terminal being connected to a tapping between the resistances 92b and 93b. The invention is particularly applicable to the measurement of potentials in very high resistance circuits, such as those used in determining the pH values of solutions, and an arrangement for this purpose is shown in Fig. 1. A test cell 1 comprising a glass electrode 3 and a calomel electrode 5 immersed in the solution to be tested is connected by a conductor 9 to the front contact 11 of a transfer switch 13, the movable contact 15 of which is connected to the control grid 21 of a screen grid tube 23, with an anode 29 connected through a bias battery 37 to the grid 39 of a triode 41. The anode 45 of the latter is connected through a milliammeter 47 to the positive side of a battery 35, the anode 29 of the valve 23 being connected to the same side of the battery through a high resistance 33. The cathodes of the two tubes are connected in series with each other and a battery 49 through a control switch 51, the cathode 25 of the tube 23 being shunted by a resistance 53. The circuit from the grid 21 is completed back to the cathode 25 either through the front contact 11 of switch 15, the cell 3 and potentiometer 55 or through the back contact 17 of switch 13 and through a switch 67 to a grid return lead 72 which connects to a sliding tap 73 on a resistance 75 connected in series with a resistance 77 and a variable resistance 79, all three resistances being in shunt to the cathode 43 of tube 41. Another resistance 8 is shunted between the top 73 and the junction point of resistances 75 and 79. The arrangement enables a variable bias to be placed on the grid return lead to compensate for variations in the strengths of the batteries, the general circuit arrangements being similar to those described in Specification 468,138. The potentiometer 55 is adjusted until the milliammeter 47 gives the same reading in either position of the switch 13 ; the potentiometer then indicates the value of the potential of the cell 1. The battery 91 of the potentiometer is preferably a dry cell, and since the E.M.F. of this is greater than the total range of potentials required to be measured in determining the pH values, the battery 91 is connected to the resistance 89 through resistances 95, 97, the latter being variable. The potentiometer is standardized against a standard Weston cell 99 by means of the switch 67. The E.M.F. developed by a glass electrode-calomel electrode cell varies in general with the temperature of the solution, but is constant at the particular value pH = +0À1. The temperature effects may be compensated by connecting tapped resistances in series with the resistances 93 and 95, and adjusting the tappings simultaneously in accordance with the temperature. In the arrangement shown in Fig. 1, a tapped resistance 103 is connected in series with the resistance 95, and one end of the resistance 92 is connected to a tap 110 on a resistance 90 shunting the resistance 89, the tap 110 corresponding to the pH value of +0À1. A switch 106 is set to the tapping on the resistance 103 corresponding to the temperature of the solution. A mathematical analysis of the potentiometer is given in the Specification. In order to reduce the possibility of current leakage from the elements 3, 9, 11 and 15 the grid return lead 72 is grounded to the frame of the instrument instead of grounding the cathodes. Swinging of the needle of the milliammeter is reduced by connecting a condenser 131 across the grid circuit of the valve 43. Various methods for adjusting the sensitivity of the amplifier are described; in the arrangement shown a variable resistance 137 is connected in series with the screen grid 27 of the input tube 23. Errors resulting from grid current in the input tube are compensated by connecting a resistance 57, of approximately the same value as the resistance of the glass electrode 3, in series with the back contact 17 of the switch 13.